Current methods of raising commercial livestock include housing a large number of animals within a confined space. Numerous drawbacks of such confinement include, but are not limited to, the significant amount of waste which must be removed periodically to ensure adequate sanitation and to prevent disease and the hydrogen sulfide in the ambient air within the animal enclosure.
The most common commercial waste removal method is to flood the housing area with water to wash away the waste comprising urine and manure. The resulting waste, that being a liquid/solid manure slurry, can be directed to a tank, or more commonly a manmade surface impoundment or lagoon, where the solids and particulates settle to the bottom and the waste decomposes predominantly under anaerobic conditions
Anaerobic organisms can degrade many waste constituents without the added expense of mechanical aeration and complete mixing. This type of treatment commonly produces methane gas and sulfides, particularly hydrogen sulfide. Hydrogen sulfide emitting from uncovered anaerobic treatment systems will create foul odors and can also be a safety hazard in confined areas. The methane gas generated could be captured as a fuel if the treatment unit is covered, but the gas is normally still contaminated with hydrogen sulfide which must be reduced to acceptable levels if the methane is to be used for energy generation.
Numerous industrial processes generate hydrogen sulfide which can be captured by absorbers/scrubbers to produce non-volatile compounds such as sodium bisulfide. Crude natural gas and biogas frequently contain hydrogen sulfide as a contaminant which can be removed by absorption. Hydrogen sulfide can also be an ambient air pollutant which can be present in industrial settings and in environments where anaerobic degradation of organic compounds is occurring, such as concentrated animal feeding buildings and pits.
Absorption liquids containing sulfides are generated when a gas containing hydrogen sulfide is processed in an absorber or scrubber where it contacts an aqueous liquid to absorb the hydrogen sulfide. While hydrogen sulfide has some solubility in water, the amount of gas dissolved is limited. Typically, the aqueous liquid will contain an alkali to convert the hydrogen sulfide into a water-soluble bisulfide or sulfide ion to greatly enhance the rate and amount of hydrogen sulfide that is absorbed into the liquid. The base can be added as a single charge or can be added throughout the absorption as alkalinity is consumed by the hydrogen sulfide. In some instances, gases other than hydrogen sulfide may be in the gas stream to be absorbed. If the gas happens to be an alkaline gas, such as ammonia, the amount of base added (if any) to the absorption liquid can be reduced.
One method of eliminating sulfides is to oxidize them to a new compound that is not malodorous or toxic. The oxidation of sulfides in aqueous liquids can be accomplished chemically with oxidizing agents such as hydrogen peroxide, chlorine dioxide, hypochlorite salts, methylmorpholine-N-oxide, nitrate/nitrites. These methods are effective but have drawbacks which can include high chemical costs, handling of hazardous chemicals and formation of unwanted by-products. Oxidation can be accomplished biologically, but this is usually expensive and can produce odors in the treatment units. A third method is oxidation with molecular oxygen in the presence of a catalyst. Sulfides may also be treated by other methods such as absorption or sequestering.
The most common catalyst for sulfide oxidation is a chelated metal catalyst, most particularly iron chelated by an aminopolycarboxylic acid. The normal product of oxidation with this catalyst in aqueous fluids is elemental sulfur which precipitates. The catalyst is typically regenerated with molecular oxygen, normally atmospheric air which can also degrade the catalyst. This method for oxidizing sulfides is not without drawbacks. It requires removal of solid elemental sulfur and replenishment of catalyst.
There is a continued need for a simple and efficient method of removing sulfides from a waste stream, for example, from wastewaters comprising biologically degradable constituents that produce sulfides. There is also a need for a process to reduce the level of hydrogen sulfide in ambient air within animal enclosures without generating a hazardous byproduct.